X-ray imaging is of high importance in view of its numerous applications such as, for example, security screening, medical imaging, quality inspection and non-destructive testing. With growing need for better resolution, accuracy, and improved frame rate of the generated X-ray image sequence, various techniques have been employed thus far including X-ray imaging interferometry and dual-energy radiography.
Publications concerning interferometry techniques are listed below. Patent application EP1731099, entitled “Interferometer for quantitative phase contrast imaging and tomography with an incoherent polychromatic x-ray source” discloses an x-ray interferometer arrangement comprising only one phase grating (122) and absorption grating (123). This interferometer can be used to obtain phase contrast images with a standard x-ray tube. Additionally, the new type of interferometer may use a source consisting of an array of individual sub-sources. Each of the sub-sources is individually coherent but mutually incoherent to the other sub-sources. The array of sub-sources may be generated by placing an array of slits, i.e. an additional amplitude grating (121) close to the source.
Pfeiffer et al. show in the article entitled “Hard x-ray phase tomography with low-brilliance sources”, published in Phys. Rev. Lett. 98, 108105 (2007), that a setup consisting of three transmission gratings together with appropriate tomographic filtered back projection (FBP) algorithms can yield quantitative 3D information of the real and imaginary part of the refractive index distribution of macroscopic objects. The setup can be used with conventional X-ray tube sources with square millimetre source sizes and several kW power.
Pfeiffer et al. disclose in the article entitled “Phase retrieval and differential phase-contrast imaging with low-brilliance X-ray source”, published in Nature Physics 2, 258-261 (2006), a setup consisting of three transmission gratings can efficiently yield quantitative differential phase-contrast images with conventional X-ray tubes. In contrast with existing techniques, the method requires no temporal coherence, is mechanically robust, and can be scaled up to large fields of view.
Patent application WO2007074029, entitled “Focus detector arrangement for generating phase-contrast x-ray images and method for this”, discloses a focus detector arrangement of an X-ray apparatus for generating projective or tomographic phase-contrast images of an examination object, wherein a bundle of spatially coherent X-rays is generated by an anode that has areas of different radiation emission arranged in strips and extending parallel to the grid lines of the phase grid of the focus detector arrangement. In addition, the invention also relates to a method for generating projective or tomographic X-ray phase-contrast images of an examination object with the aid of such a focus detector arrangement, wherein a bundle of coherent radiation is generated by an anode that has areas of different radiation emission arranged in strips and extending parallel to the grid lines of the phase grid.
Patent application WO2004071298, entitled “Apparatus And Method To Obtain Phase Contrast X-Ray Images” discloses an apparatus for generating a phase contrast x-ray image comprising in an optical path as seen in the direction of the light flow: a) an incoherent x-ray source; b) a first beam splitter grating for splitting the light beams of said x-ray source; c) a second beam recombiner grating for recombining the splitted beams in a recombination distance from the second beam recombiner grating; d) an optional third analyzer grating in order to offer an adsorption lines grating matching the interference lines downstream of said second beam recombiner grating in an analyzer plane (a); e) an image detector disposed downstream of the analyzer plane (a); and f) a means for introducing a sample into said optical path upstream or downstream of the second beam recombiner grating.
Patent application EP1623671, entitled “X-Ray Imaging System and Imaging Method”, discloses an X-ray imaging apparatus equipped with first and second diffraction gratings and an X-ray image detector. The first diffraction grating is constructed to generate the Talbot effect in the X-rays diffracted by the first diffraction grating. The second diffraction grating is configured so as to diffract the X-rays diffracted by the first diffraction grating. The X-ray image detector is configured so as to detect the X-rays diffracted by the second diffraction grating. By diffracting X-rays diffracted by the first diffraction grating, the second diffraction grating is capable of forming image contrast caused by changes in phase of X-rays due to the subject arranged in front of the first diffraction grating or between the first diffraction grating and the second diffraction grating. The X-ray image detector is capable of detecting X-rays creating image contrast.
Patent application US20050190882 to McGuire, entitled “Multi-spectral x-ray image processing” discloses a method of performing x-ray analysis on a body of unknown composition. The method comprises bombarding the body with a plurality x-ray beams, each x-ray beam having unique line spectra; and determining the compositional makeup of the body by detecting and analyzing x-rays reflected off of the body. Referring now to FIG. 1A, the above-referenced publications concerning interferometry-based imaging techniques are implemented by an interferometer device 100 that includes, inter alia, three different gratings and more specifically, a source grating 121, a beam splitter grating 122 and an absorption grating 123. Beam splitter grating 122 may have a grating as is schematically illustrated in FIG. 1B, wherein the height H of the structure may be such that the relative phase shift of X-rays passing through beam splitter grating 122 causes a phase shift on the X-rays of Δφ=π or an odd multiple thereof. Beam splitter grating 122 may thus also be referred to as phase grating 122. Moreover, if beam splitter grating's 122 duty cycle, which is defined as B/p equals 0.5, i.e., the width of the grating bars are equal to the width of the spaces, is illuminated by an X-ray plane wave (of wavelength λ) the incoming X-ray wave field is essentially divided into the two first diffraction orders. Since these two diffracted beams overlap almost completely an interference pattern is formed that changes depending on the distance d downstream of beam splitter grating 122. At some specific distances dn=(n/16)*DTalbot (n=1, 3, . . . ), known as the fractional Talbot distances, a periodic pattern of linear fringes is observed. These fringes enable interpreting phase, as is for example outlined in patent application EP1731099. DTalbot is defined as the distance away from the initial wave front profile along the optical axis at which any laterally periodic wave front profile is replicated. If the grating, which forms the initial wave front profile, has periodicity P and wavelength λ, the Talbot distance may be found at DTalbot=2p2/λ. The phase-shift of an x-ray with energy E passing through a layer of thickness H is determined by the material's refraction index. The phase shift Δφ can thus be considered to be proportional to the material's thickness and the incident wavelength. Therefore, to achieve the required relative phase-shift of Δφ=π the depth of an appropriate grating increases with increasing x-ray energy. For example, in order to achieve a phase shift of Δφ=π in Si for X-ray at energy of 28 keV, the structure depth required in Si material has to be about H=35 μm and increases linearly with higher incident X-ray energy, as is schematically illustrated in FIG. 2. It should be noted that adjective and adverb “linear” and “linearly” also encompass the meaning “substantially linear” and “substantially linearly”.Towards higher X-ray energies not only deeper structure depths, but also smaller grating periodicity and thus higher aspect ratios (aspect ratio: structure depth H divided by the periodicity p) are required. Since the Talbot distance DTalbot and thus the dimension of the whole set-up scales according to the equation for the Talbot distance with p12, the periodicity determines the compactness of the entire set-up and the dimensions of the gratings. Generally, an increase in the periodicity and/or structure depth of gratings renders the fabrication process of the gratings more difficult.
Referring now to conventional radiography a thick sample of a poorly absorbing material can come out in the same grey level in the radiograph as a thin but strongly absorbing one (known as the “overlapping problem” in radiography). In other words, the products of absorption coefficient (μa) and travelling path length (μa*Δx) through the sample are equal for both cases. In order to achieve elemental identification of the material under investigation many applications in conventional x-ray radiography thus use a set-up in which individual images are detected for two different energy ranges, i.e., dual energy radiography. Since the absorption coefficient μa scales with the electron density, the ratio of a first low energy (LE) and a second high energy (HE) measured intensity downstream an imaged object correlates with the energy of the imaging X-ray radiation incident on the object. Therefore, the absorption coefficient of the object being imaged and thus the object's material can be determined. The knowledge of electron density can then serve as an evidence for the material's (elemental) composition. As an example x-raying systems at airports use this technique in order to reliably detect potentially harmful metals and other illicit materials in the luggage.
Another method of X-ray imaging is known as Dual Energy X-ray Absorptiometry (DEXA) that is applied, for example, to determine bone density in people to detect thinning bones or osteoporosis.
Other methods than DEXA include tomography scans, which may yield similar results. However, compared to dual X-ray imaging, tomography scans may be time consuming, and the expended dose may be higher.
In general, dual X-ray imaging methods may implemented by so-called pseudo dual energy systems that may employ a single anode, a first and a second detector, wherein the two detectors are positioned in alignment to each other. The system further includes an absorber positioned between and in alignment with the two detectors. The first detector is implemented by a thin scintillator creating the image of the low energy part of the spectrum. The absorber (typically made of Cu or Al) absorbs the low energy tail of the incident X-ray. Therefore, the X-ray emanating from the absorber and incident on the second detector is the HE part of the X-ray. Accordingly, the image obtained from the second detector represents the image HE part of the X-ray. Instead of using the above-outlined setup to enable the pseudo dual-energy X-ray imaging method, dispersive digital image detectors may be employed, wherein detection thresholds for the X-ray energy is selectable.